Alloy



Patented July 12, 1932 PATENT OFFICE RICHARD E. BISBELL, OF CLEVELAND, OHIO, ASSIGNOR TO THOMPSON PRODUCTS, INC

' OF CLEVELAND, OHIO, A CORPORATION OF OHIO ALI BOY ll'o Drawing.

This invention relates to alloys having high resistance to corrosion by hot gases and also possessing considerable hardness which is argely retained at a dull red heat.

I have found that such advantageous properties may be obtained to a large degree by means of additions of cromium, nickel and silicon coupledwith a special heat treatment as described in my co-pending application, Serial No. 252,913, filed June 3, 1930, which has issued into Patent No. 1,761,866.

One of the principal objects of the present invention is to provide alloys having improved chemical and physical characteristics.

I have found that cobalt may be used in place of nickel. However, whenthe proportions of cobalt are those given for nickel in the above mentioned application, the alloy is normally martensitic and of a low transformation point. This transformation point may be lowered by increasin the cobalt content or raised or voided by a ding aluminum.

So far as the effect on the transformation point is concerned, cobalt is less effective than nickel. So far as imparting resistance to oxidation and corrosion by hot gases is concerned, cobalt is about equal to nickel.

I have found further that manganese may be used to replace a part of the cobalt or nickel so far as obtaining a normally austenitic structure is concerned.

My object is to attain an alloy which will not change its structure to an marked ex-- tent during use. This may be one in one or other of two ways, one by depressing the austenitic-martensitic change point, another by raising the Ac, point. The former is brought about by adding such elements as manganese, the latter by addition of aluminum.

Application filed September 4, 1928. Serial No. 303,973.

Of the physical characteristics, one of the most desirable for such purposes as exhaust valves is capacity to withstand heating to temperatures under 1500 F. and; cooling sults frequently in a change of hardness and I also warpage or cracking of the articles made from such alloys.

Even if substantial change of hardness or warpage do not occur, it is desirable to eliminate as 'far as possible all volume changes due to changes ininternal, structure and not to normal thermal expansion.

Substantial additions of chromium and nickel or cobalt with. or without smaller amounts of manganese, give a steel which may be heated to 1500 F. and cooled without great volume changes as indicated by the dilatometer or similar means. However, there is a critical oint around 1250 F. which is evidenced by a volume change in the case of an alloy having the composition set forth in my above mentioned application when in the martensitic condition. Similarly, a chromium cobalt-manganese-silicon alloy, containing about 14% chromium, 8.5% cobalt, 3% manganese,3.5 silicon and 35% carbon, has a critical point when in the martensitic condition, below 1500 F. which may be raised or voided by the addition of aluminum.

A further object of this invention, therefore, is-to provide an alloy which has the advantages of chromium-nickel steels without the disadvantage of a critical or volume change point below 1500 F. when in the martensitic form. a

I have found that this critical point can be eliminated by the addition of aluminum in relatively small amounts.

Two suitable compositions of my improved alloy are as follows:

high nickeL' High With or without aluminum, alloys of the above compositions are normally soft and austenitic in structure. Ordinary heat treatment produces little hardening effect, and it is necessary to give the alloys a special treatment to give them a hard martensitic structure.

The preferable heat treatment for alloys of this character is a prolonged soak for four hours at 1650 F., followed by a slow cooling at F. per hour to 1350 F., and finally air cooled from that temperature.

Preferably, the elements which affect the transition of the iron from the gamma or austenitic form into the alpha or martensitic form, are present in an amount sufiicient to prevent martensitic hardening with normal heat treatment and below that amount which will prevent such hardening by prolonged soaking at high temperatures.

So far as the effect on the change points of the alloy are concerned, nickel, cobalt, and manganese, all lower the austenitic-martensitic change point, although the amounts of each required to produce similar results vary considerably. That is not the case when the resistance to oxidation by hot gases or the like is considered. Nickel tends to lower the resistance to oxidation noticeably with chromium 9%, and to a much lesser degree above 9%. Chromium increases the resistance to oxidation. Nickel and cobalt, however, have the desirable characteristic of increasing the red-hardness of the alloy, whereas steel havin chromium alone in large proportions has re atively little red-hardness. V

Silicon and aluminum both increase surface stability in that a thin scale forms, which neither peels nor cracks off nor increases in thickness, and are, therefore, to that extent interchangeable. Hence, where nickel is used, and a high degree of both red-hardness and surface stability are required, high silicon and/or aluminum should accompany aluminum, however, tends to make the alloy brittle'and difiicult to work. I

While it is desirable to use all of the above mentioned elements as ingredients of the alloy, very useful results can still be obtained by omitting one or more of them. Specifically chromium-cobalt-carbon-iron alloys have many desirable qualities. The same is true of the following combinations of elements other than iron and carbon; chromium, nick el or cobalt and aluminum; chromium, nickel or cobalt and manganese with or without silicon and/or aluminum; chromium, nickel or cobalt and silicon and aluminum, as well as others.

I am also aware that numerous changes in proportions of the ingredients of my alloys may be made without departing from the spirit of my invention, and I do not desire to limit the patent otherwise than as necessitated by the prior art.

I claim as my invention:

1. An alloy having a martensitic structure, consisting principally of chromium,cobalt,silicon. carbon and iron in which the chromium is approximately 14 1, cobalt, about 8.5%, silicon about 3.5% and carbon about .25 to 35%, said alloy being heat treated for about four hours at about 1650 F., followed by a slow cooling at about 50 F. per hour to about 1350 F. and finally being air cooled to room temperature.

:2. An alloy having a martensitic structure, and consisting principally of alpha iron, chromium. cobalt, silicon and carbon, in which the chromium content is about 14%, the cobalt approximately 8.5% and the silicon about 3.5% said alloy being heat treated for several hours at about 1650 F. and then slowly cooled to room temperature.

' 3. A ferrous alloy having a martensitic structure, and consisting principally of iron, cobalt, chromium and aluminum, the chro- 100 mium content being about 14%, cobalt of about 8.5%, and the aluminum about 1.8%, said alloy being heat treated for several hours at about 1650 F. and then slowly cooled to room temperature.

In testimony whereof I have hereunto sub scribed my name at Cleveland, Cuyahoga County, Ohio.

RICHARD E. BISSELL. 

